The present invention relates to a device for die-casting metal material, comprising a screw unit for bringing the material into a thixotropic state and a cylinder/piston unit fed by the screw unit for applying pressure to the thixotropic material for the die casting. The invention further relates to a system for die-casting metal material in the thixotropic state, comprising such a device, and to a method for die-casting metal material in the thixotropic state with use of such a device.
When die-casting metal material in the thixotropic (“semi-solid”) state, also referred to as “metal injection moulding” and known for example under the trade name Thixomolding®, diecast parts can be produced which have properties that are improved compared with parts formed in the conventional die-casting process. Here, the materials have to be brought to the transition temperature between the solid and the liquid phase, such that distributed crystallised constituents are embedded in contiguous melted regions (“thixotropic phase”). Due to the additional influence of shear forces, the crystalline structures of the solid constituents are reduced in size, and the viscosity of the material falls, which facilitates the injection of said material into the die-casting mould and allows precise die casting.
Metal injection moulding machines are known for example from EP 0 080 787. In accordance with this known prior art, metal material is heated in the screw chamber of a combined cylinder/reciprocating screw unit and is exposed to a shear load by rotating the reciprocating screw so as to bring the material into the thixotropic state. The rotation of the reciprocating screw simultaneously conveys the material from the screw chamber into the injection chamber of the cylinder/reciprocating screw unit arranged in front of the reciprocating screw, wherein the reciprocating screw retreats progressively in the cylinder. If a quantity of thixotropic material sufficient for the die casting is located in the injection chamber, the material is injected into a casting mould by applying pressure to the reciprocating screw by means of a hydraulic system. In order to prevent the thixotropic material from flowing back from the injection chamber into the screw chamber as a result of the high pressure during the injection, the tip of the reciprocating screw is equipped with a check valve. Such a valve is exposed to high loads as a result of the friction between the reciprocating screw and the cylinder wall, the high process temperatures inside the cylinder and the application of pressure. Due to its arrangement on the reciprocating screw, an uncontrolled valve is generally used, which leads to a loss of accuracy. The required short injection times in conjunction with the large mass of the reciprocating screw also place high demands on the hydraulic system and control elements thereof due to the inertias in conjunction with the mass accelerations.
DE 190 79 118 discloses a metal injection moulding machine which achieves a much smaller piston mass due to the separation of the screw unit and cylinder/piston unit. In such a machine the material is first brought in a screw unit (screw extruder) into the thixotropic state and thereby fed to a separate cylinder/piston unit, which performs the injection procedure. Here, the screw unit conveys the thixotropic material via a hot runner into a feed chamber in the cylinder arranged behind the piston. For injection, the piston is first retracted, the material passes through a check valve in the piston from the feed chamber into the injection chamber on the other side of the piston, and is injected into the die-casting mould as a result of the application of pressure to the piston. In this method too, the valve located in the piston is exposed to high loads; in addition, the piston moved during the injection procedure with closed check valve causes an undesirable, uncontrollable suction in the screw extruder via the hot runner.
A method is known from WO 2011/116838, in which a semi-solid metal strand is produced in an extruder and is transferred in portions by means of tongs into the feed chamber of a separate cylinder/piston unit.
US 2002/0053416, as an alternative, presents a direct feed from the screw unit via the hot runner into the injection chamber of the cylinder/piston unit. In such a configuration, the high injection pressure of the cylinder/piston unit passes directly back into the screw unit via the hot runner. This results in a backflow of thixotropic material into the screw unit to an extent that is virtually impossible to control, and consequently leads to injection of an undefined quantity of material into the die-casting mould with negative effects on the quality of the diecast part. At the same time, the screw unit and the overall mechanical and hydraulic systems thereof are subjected to repeated high pressure surges, which, in addition to the direct loading, also increases the level of wear.
WO 01/021343 describes a method for bringing a liquid metal alloy into the thixotropic state in a twin-screw extruder and for subsequent injection moulding by means of a cylinder/piston unit. The twin screw, by means of a very high rotational speed, is intended to limit a deposition and curing of the melt on the cooled wall of the extruder, and a controllable mechanical valve on the extruder is intended to prevent the melt from flowing out prematurely into the cylinder/piston unit. Such a valve with moving parts is highly stressed and at risk of failure due to the ongoing temperature fluctuations and the mechanical loads.